Several types of marine vessels are equipped with an opening in the base or bottom of the hull, that provides access to the water underneath the vessel. Typically, this opening extends from a deck on the vessel, through the hull, and to a portion of the hull bottom. An opening of this type is commonly referred to as a “moon pool”.
Moon pools are common in floating platforms that are used for drilling, production and/or maintenance of subsea oil and gas wells and associated subsea equipment. Drill strings, completion equipment, and various other tool and devices, are lowered into the water through the moon pool.
Moon pools are also common in drill ships and in diving support vessels and other offshore support vessels, including ships that are used for deploying and controlling subsea remotely operated vehicles (ROVs).
The relative movement between the water and the vessel generates turbulent flow in and around the moon pool; particularly if the vessel is a ship moving though the water at cruising speeds. These turbulent flows may be very strong and hence be the source of considerable drag forces acting on the hull. The industry is therefore seeking means to reduce the turbulence, and hence the drag forces, caused by the ship's moon pool.
Another disadvantage with open moon pools is the so-called “piston effect”, in which the oscillating seawater column inside the moon pool generates a forced airflow out of and into the moon pool. This airflow may be uncomfortable and even dangerous to personnel in the vicinity of the moon pool. Another problem with open moon pools on vessels operating in arctic waters is interference with ice floe and ice chunks. These objects may enter the moon pool where they could cause damage to equipment and structures, and jeopardize operations through the moon pool.
An overview of the prior art is provided in the following.
U.S. Pat. No. 6,503,022 B1 describes a vessel having a moon pool positioned in the centre of the vessel. A moon pool plug is adapted to have controllable positive and negative buoyancy. The plug has a top, a cover, a bottom, a first angled side, a second angled side, a third angled side, a fourth angled side; first plates disposed traversely across the bottom of the plug; and second plates disposed perpendicular to said first plates across the bottom of the plug. There is a water valve for permitting sea water to egress from the plug; an air valve for permitting air to be pumped into the plug. There is a guiding device secured to the plug to guide the plug into the moon pool. There is a hoist secured to the plug to lift the plug into the desk of the vessel.
US 2014/0202371 A1, discloses a drillship having a vortex suppression block with a recessed flow stabilizing part in a moon pool. The flow stabilizing section is formed so that a bottom of the vortex suppression block that protrudes from a bottom of a hull toward a stern in the moon pool is recessed in an upward direction of the hull to stabilize a flow in the moon pool and reduce a variation in resistance of the drillship. The flow stabilizing section is formed by partly recessing the bottom of the vortex suppression block toward an upper portion of the hull.
U.S. Pat. No. 8,770,124 B2 discloses a device for diminishing flow resistance in a moon pool. The device includes a guide structure having a lattice shape. The guide structure includes horizontal guide plates, guide ramps, longitudinal reinforcing beams and a hinge axis. The horizontal guide plates are spaced apart from each other along a longitudinal direction of a hull and arranged in rows along a lateral direction of the hull. The guide ramps are arranged in rows along the lateral direction and coupled to the respective horizontal guide plates. Front ends of the guide ramps are inclined upwards towards a bow side of the hull and spaced apart from rear ends of the adjacent fore horizontal guide plates. The longitudinal reinforcing beams support the horizontal guide plates and the guide ramps. The lattice guide structure is rotated around the hinge axis towards a transit position or a working position.
KR 2010 0069983 discloses an open-type door for reducing resistance of a moon pool of a drilling ship and a marine structure is provided to minimize damage to due to vibration. The open-type door comprises first and second doors which are installed on two sides of the bottom of the moon pool. One end of the first and the second door is connected to the hull by a hinge connected to a motor through a chain. The hinge connected to the chain is rotated together with the motor, thereby opening and closing the doors.
KR 2010 0020613 discloses a resistance reduction apparatus for a moon pool, to reduce vortex flow caused in the moon pool by effectively interrupting a flow field into the moon pool. A first rod and a second rod are located separate from each other at the entrance of a moon pool. A grid is located between the first rod and the second rod, with both ends connected to the first and the second rod. The first rod is fixed to one side of the moon pool entrance and the second rod can move close to the first rod.
JP H 08 119190 discloses a device to reduce resistance by generation of eddy currents in a lower end aperture part of a moon pool for a floating structural body having a moon pool formed of cylindrical surrounding walls penetrating from an aperture edge of an upper deck to an aperture edge of an outer plate of a bottom part. In an aperture part at a lower end of a moon pool formed of cylindrical surrounding walls penetrating from an aperture deck of an upper deck to an aperture edge of an outer plate of a bottom plate, a cover-shaped structural body pivotally attached through a hinge to be opened/closed is composed of a number of vertical plates to part the inside of the aperture part combined like a grid in such a way that generation of eddy currents can be prevented while allowing going in/out of water at the aperture part in the closed condition of the structural body in navigation.
JP S 61 132495 discloses a device for improving propulsion efficiency, by installing a horizontal roll wound with a canvas at the lower edge of one inner wall surface of a moon pool and freely laying the canvas towards the opposed inner wall surface and closing the moon pool during navigation. A horizontal roll which can be revolved normally and reversely is installed at the lower edge of one inner wall surface of a moon pool, and a canvas is wound. A wire for pulling in the horizontal direction towards the opposed inner wall surface is engaged with the outer side edge part of the wound canvas, and connected to a taking-up apparatus on a deck, and the canvas can be laid towards the opposed inner wall surface.
The present invention represents an improvement over the prior art devices and offer additional advantages.